The present invention relates to water-cooled marine engines, and in particular to a cooling water passage arrangement suitable for use in relatively small water-cooled outboard marine engines.
In a water-cooled internal combustion engine, the cooling water temperature is maintained at a prescribed level by providing a thermostat valve at an exit end of a water jacket formed around a cylinder block. The thermostat valve opens and closes according to the cooling water temperature as well known in the art. In case of an outboard marine engine, it is common to draw cooling water from an inlet port provided in a submerged part of the outboard engine and directly forward it to the water jacket by using a pump.
In such an engine, as soon as the thermostat valve opens as a result of a rise in the cooling water temperature and the high temperature cooling water is expelled, cooler water from the body of water surrounding the watercraft is drawn into the cooling water passage. Particularly if the engine consists of a single or inline two cylinder engine, because the passage for the cooling water for the cylinder block is relatively short, there is a tendency for the cooling water of such a small outboard marine engine to rapidly fluctuate in temperature. In the worst case, the thermostat valve may open and close in short intervals. This is called xe2x80x9chuntingxe2x80x9d, and compromises the proper control of the cooling water.
In view of such problems of the prior art, a primary object of the present invention is to provide an outboard marine engine which avoids such a hunting of the thermostat valve.
A second object of the present invention is to provide an outboard marine engine which is provided with a cooling water passage adapted to ensure a stable cooling water temperature control.
According to the present invention, such objects can be accomplished by providing a cooling water passage arrangement for a water-cooled marine engine, comprising: a cylinder block including a water jacket; a water inlet passage having a first end submerged in a surrounding body of water and a second end communicating with the water jacket; a water pump provided in association with the water inlet passage; a water outlet passage having a first end communicating with the water jacket and a second end communicating with a water outlet port; a thermostat valve provided in an exit passage defined between an outlet end of the water jacket and an inlet end of the water outlet passage; and a heat transfer portion provided in the exit passage upstream of the thermostat valve.
Thus, when the thermostat valve has opened and the cooling water expelled from the water jacket via the thermostat valve has been replaced by freshly introduced cooling water, the heat transfer portion having a certain heat capacity warms the freshly introduced cooling water so that the rapid change in the cooling water temperature at the thermostat valve can be avoided. In particular, by extending the heat transfer portion from a relatively warm part of the engine, it is possible to supply heat to the heat transfer portion so that the temperature stabilizing effect of the heat transfer portion may be maintained even after the heat stored in the heat transfer portion has been expended. For instance, the heat transfer portion may extend directly from a wall member having a threaded hole formed therein for receiving a threaded bolt for securing a cylinder head to the cylinder block.
To enhance favorable transfer of heat to the heat transfer portion and ensure a reasonable amount of heat capacity to the heat transfer portion without unduly increasing the resistance to the flow of the cooling water, the heat transfer portion may extend along a certain distance along a flow direction of the exit passage and across the exit passage. Alternatively, the heat transfer portion may extend along a certain distance along a flow direction of the exit passage and project into the exit passage.
According to a preferred embodiment of the present invention, to simplify the manufacturing process, a mating surface of the cylinder block for a water jacket cover to define a cooling water outlet passage, a mounting surface for the thermostat valve at an outlet end of the exit passage, and a mounting surface for a flush valve are defined by a common machined surface of the cylinder block.
To ensure a favorable access to the thermostat valve and/or flush valve, the common machined surface of the cylinder block may be defined on an outer side of an exhaust passage of the engine with respect to a cylinder of the engine.